In order to make deep-sea reservoirs or marginal fields sufficiently cost-effective, oil companies have to develop new development techniques, as economical as possible. It is thus more advantageous to directly transport the two-phase mixture consisting of liquid (oil and a little water) and gas in a single pipe or pipeline to onshore facilities in order to be separated. A pipe portion greatly inclined to the horizontal (often close to the vertical), referred to as riser by specialists, which is connected to the deep-sea pipe, is used therefore. However, the gas and the liquid being transported together, flow instability phenomena may occur in the zone of connection with the riser, which lead to serious development problems.
In particular, when the gas and liquid inflow rates are low, the liquid phase accumulates in the lower parts of the pipeline and stops the gas flowing past. The upstream pressure increases and eventually expels the liquid slug to another low part or even in the phase separator at the outlet. These accumulation phenomena can reduce the productivity and fill pipes designed to receive gas with liquid. One of these phenomena, commonly referred to as severe slugging by specialists, has formed the subject of many studies, either experimental by means of test loops, or by simulation by means of simulation softwares such as, for example, the TACITE simulation code which is notably the object of the following patents or patent applications: U.S. Pat. No. 5,550,761, FR-2,756,044 (U.S. Pat. No. 6,028,992) and FR-2,756,045 (U.S. Pat. No. 5,960,187), FR-00/08,200 and FR-00/09,889 filed by the applicant.
This slugging phenomenon is described hereafter in the simple case illustrated in the accompanying figures where a pipe of low inclination and a riser ended by a separator designed to separate the liquid phase from the gas phase are considered.
The liquid accumulates in the lower part of the pipe and tends to stop the gas flowing past. The gas is compressed until the upstream pressure exceeds the pressure due to the weight of the accumulated liquid. A long liquid slug is then pushed by the expanding gas. Under such conditions, an alternating phenomenon is observed, where the liquid blocks the gas phase, then flows off under the pressure of the gas and eventually accumulates and blocks the gas again.
More precisely, the periodic process takes place as follows:
Stage I: the liquid accumulates at the foot of the riser and stops the gas flowing past. The pressure rises;
Stage II: the upper level of the liquid having reached the top of the riser, the liquid phase flows into the separator;
Stage III: the gas pocket reaches the foot of the riser and flows into the riser. The slug flows into the separator with a much higher velocity; the gas pocket <<explodes>> in the riser;
Stage IV: when the gas pocket reaches the top of the riser, the pressure at the foot of the pipe has a minimum value. The liquid falls down along the wall of the riser. It accumulates again at the foot of the riser and a new cycle starts.
A well-known technique referred to as gas lift by specialists allows to overcome this phenomenon. It essentially consists in permanently injecting gas at the base of the riser to prevent the accumulation of liquid at the bottom of the pipe. Since this phenomenon cannot be really controlled, most of the time one is led to inject large amounts of gas, which requires considerable compression means. Furthermore, injection of large amounts of gas substantially modifies the gas-oil ratio (GOR), which complicates the phase separation operations at the top of the riser.